Is it the 'loudness' you want to increase, or the 'noticability'?

If the latter, and the loudness as as good as it gets (ie. the MP3 is
already normalised to 100%) you might be better off tinkering with the
frequency spectrum of the ringtone. I'm guessing that the higher
frequencies get better reproduced by the Android 'speaker' than the
lower, so performing the equivalent of a 'turn the treble up' via
Audacity or similar might improve matters for you.

I also suspect that there are cleverer tricks to improve the
noticability, along the lines of simulating a clipping transducer, but
I don't know for sure.

I have a fairly low-frequency ringtone on my ancient Nokia, and it's
not very noticable. I keep meaning to do something similar with
that...

HTH
J^n